1. Field of the Invention
The present invention relates to a method for generating subsurface seismic waves, and to a source for carrying out said method. More particularly, the invention relates to using a fluid and a blocking body to generate seismic waves.
2. Description of the Related Art
One method and source for using a fluid and a blocking body to generate seismic waves is taught in U.S. Pat. No. 3,993,974. The method identified in that patent comprises:
inserting a blocking body into a fluid supply conduit, arranged subsurface and conducting a fluid downward; PA1 enabling the fluid flow to carry said blocking body downward and forcing said body against a seat in a subsurface supply conduit section, thereby blocking the fluid; and PA1 continuing the fluid flow, thereby rising the fluid pressure until said body is forced through said seat, resulting in generating seismic waves.
In particular, a frangible object is inserted into the drill pipe arranged in a borehole being drilled. Subsequently said object is carried down through said pipe by the mud stream until the opening from the pipe to the bit is reached. Then the opening is blocked. By said blocking, the pressure inside the pipe will rise to a certain level at which the object will shatter and crumble into many pieces. Thereupon the compressed liquid will explosively expand through the openings in the bit in shock wave fashion and elastic or seismic waves are generated and are subsequently propagated through the subsurface formation.
Although, for example, geophones at the earth's surface will pick up signal components from said seismic waves, said signals, however, are substantially spread in time by said shattering and crumbling. This will cause unclearness both as to departure/arrival features of said waves and the ability to distinguish said signals from bit signals and further noise. Moreover, signals thus obtained are not reproducible. It may be clear that such signals are very difficult to be handled in order to arrive at unique subsurface formation mapping.